Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 37–45
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Silver and gold nanoparticles for sensor and antibacterial applications M.R. Bindhu, M. Umadevi ⇑ Department of Physics, Mother Teresa Women’s University, Kodaikanal 624101, Tamil Nadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Ag and Au nanoparticles synthesized
using Solanum lycopersicums extract as reducing agent. Shows good antimicrobial activity. Act as very good copper and iron sensors. Can be used in water purification processes.
a r t i c l e
i n f o
Article history: Received 28 November 2013 Received in revised form 4 February 2014 Accepted 19 February 2014 Available online 12 March 2014 Keywords: Silver nanoparticles Gold nanoparticles Solanum lycopersicums Sensing activity Antimicrobial activity
a b s t r a c t Green biogenic method for the synthesis of gold and silver nanoparticles using Solanum lycopersicums extract as reducing agent was studied. The biomolecules present in the extract was responsible for reduction of Au3+ and Ag+ ions from HAuCl4 and AgNO3 respectively. The prepared nanoparticles were characterized by UV–visible spectroscopy (UV–vis), Fourier transform infrared spectroscopy (FTIR), Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) technique to identify the size, shape of nanoparticles and biomolecules act as reducing agents. UV–visible spectra show the surface plasmon resonance peak at 546 nm and 445 nm corresponding to gold and silver nanoparticles respectively. Crystalline nature of the nanoparticles was evident from TEM images and XRD analysis. TEM images showed average size of 14 nm and 12 nm for prepared gold and silver nanoparticles respectively. FTIR analysis provides the presence of biomolecules responsible for the reduction and stability of the prepared silver and gold nanoparticles. XRD analysis of the silver and gold nanoparticles confirmed the formation of metallic silver and gold. The prepared gold and silver nanoparticles show good sensing and antimicrobial activity. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Optical sensor based on surface plasmon resonance for detection of heavy metals in water is one of the most sensitive methods which will be advantageous over other techniques as it will be a simple, inexpensive and fast method. In the past years a number of nanoparticles based sensor have been reported [1–4]. Recently ⇑ Corresponding author. Tel.: +91 04542241685; fax: +91 4542 241122. E-mail address: [email protected] (M. Umadevi). http://dx.doi.org/10.1016/j.saa.2014.02.119 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
green synthesized silver nanoparticles used in an optical sensor based on localized SPR for ammonia, mercury, copper and zinc detection was studied [5–7]. Water being the prime vital survival need for living beings in this world. Pure water is absolutely essential for a safe, healthy and long life. Purity of water is polluted by the existence of pathogenic bacteria and carcinogenic chemicals present in it. Presence of heavy metals such as cadmium (Cd), Zinc (Zn), mercury (Hg), arsenic (As), silver (Ag), chromium (Cr), copper (Cu), iron (Fe), and the platinum group elements in water causes health hazards. In particular the detection of Cu4+ and Fe3+ in water
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has been of great research interest due to the important role of Fe and Cu plays in the biological processes within the human body. Drinking water can be a source of Cu to humans as a result of water treatment, usage of galvanized pipes and tanks in distribution systems. So, detection and control of Cu ions in water samples is very important. Iron is usually found in its oxidized form, which is insoluble and nondegradable. As a result, low solubility of iron does not give the necessary sensitivity for detection of this metal ion in water. So, detection and control of Cu and Fe ions in water samples is very important. Microorganisms causing diseases that characteristically are waterborne, significantly contain protozoa and bacteria. The removal or inactivation of pathogenic microorganisms is the important step in the treatment of wastewater. In the present study, we have designed simple device to detect the contaminant concentration in water as well as to inhibit the growth of different pathogenic bacteria. Metal nanoparticles have been studied extensively because of their applications in areas such as optics, optoelectronics, catalysis, photography, nanostructure fabrication, chemical/biochemical sensing, photonics and surface enhanced Raman scattering [8]. Among the known nanoparticles, silver and gold has been widely studied because of their unique optical, electrical, and photothermal properties. Silver (AgNPs) and gold (AuNPs) nanoparticles can be synthesized using chemical and physical methods, which are expensive and often involve the use of toxic, hazardous chemicals which may pose environmental risks. A great deal of attempt has been put into the green synthesis of metal nanoparticles, using plants are a simple and viable alternative to chemical procedures and physical methods. Biosynthesis of nanoparticles using Daucus carota, Solanum lycopersicums, Hibiscus cannabinus leaf, Moringa oliefera flower, Murraya Koenigii leaf, mushroom, coconut oil, Macrotyloma uniflorum and Ananas comosus has been reported [9–16,7]. In this work, AgNPs and AuNPs were synthesized using S. lycopersicums fruit extract as reducing agent. Since S. lycopersicums is a commonly available fruit and good source of citric acid, malic acid and ascorbic acid [17]. S. lycopersicums also contains a variety of phytochemicals, including carotenoids and polyphenols. Lycopene, phytoene, phytofluene, flavonoids, quercetin, kaempferol and the provitamin A carotenoids beta-carotene are the important carotenoids in S. lycopersicums. Many of these nutrients and phytochemicals have antioxidant properties and in combination with lycopene may contribute to the numerous health benefits of S. lycopersicums. S. lycopersicums has anticancer properties [18]. S. lycopersicums consumption might be beneficial for reducing cardiovascular risk associated with type 2 diabetes [19] and protective against neurodegenerative diseases [20]. The disposal of the S. lycopersicums skin and its other fibrous materials is an economic waste for many food processing industries. Ramakrishna et al. reported that S. lycopersicums peels can effectively remove different contaminants in water, including dissolved organic and inorganic chemicals, dyes and pesticides, and they can also be used in large scale applications [21]. The synthesis of well dispersed silver nanoparticles using S. lycopersicums fruit extract as reducing agent has been reported [10]. In the present study, the synthesis and characterization of monodispersed AgNPs and AuNPs using fruit extract of S. lycopersicums has been described. The sensing and antibacterial activity of these prepared AgNPs and AuNPs has been also described.
collected from local supermarket in Kodaikanal, Tamilnadu, India. 100 g of washed S. lycopersicums were crushed in a mixer grinder for extraction. The extract was separated by centrifugation at 1000 rpm for 10 min to remove insoluble fractions and macromolecules. Then the light yellow extract was collected for further experiments. An aqueous solution of HAuCl4 (3 mM) was added to different concentration of fruit extract (5, 10 and 15 ml) and stirred for 5 min at room temperature (g1, g2 and g3). During the synthesis, initially becomes colorless and turned into brownish purple indicating the formation of gold colloid. Similarly, an aqueous solution of AgNO3 (3 mM) was added to different concentration of fruit extract (5, 10 and 15 ml) and stirred for 5 min at room temperature (s1, s2 and s3). Upon addition of the extract, the colorless solution changes colour from light yellow to reddish orange. The absorption spectra of the prepared nanoparticles were measured using a Shimadzu spectrophotometer (UV 1700) in 300– 800 nm range. XRD analysis of the prepared nanoparticles was done using PANalytical X’pert – PRO diffractometer with Cu Ka radiation operated at 40 kV/30 mA. FTIR measurements were obtained on a Nexus 670 FTIR instrument with the sample as KBr pellets. Transmission Electron Microscopic (TEM) analysis was done using a JEOL JEM 2100 High Resolution Transmission Electron Microscope operating at 200 kV.
Experimental details Chloroauric acid, silver nitrate, copper sulphate, ferric chloride, nickel nitrate, potassium chloride, cadmium acetate, manganous acetate, mercuric iodide, lithium hydroxide and zinc acetate were obtained from Sigma Aldrich Chemicals. S. lycopersicums fruit was
Fig. 1. Optical absorption spectra for S. lycopersicums fruit extract concentration variation study of (i) prepared gold nanoparticle (inset: colour changes of the gold colloid) and (ii) prepared AgNPs (inset: colour changes of the silver colloid) (a, b and c versus 5, 10 and 15 ml respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Results and discussion Optical studies Noble metal nanoparticles exhibit a strong absorption band in the visible region, which originates from coherent excitation of the conduction band electrons induced by the interacting electromagnetic field. UV–visible absorption spectra have been proved to be quite sensitive to the formation of silver and gold colloids because silver and gold nanoparticles exhibit an intense absorption peak due to the surface plasmon excitation. Fig. 1(i) and (ii) shows the optical absorption spectra of prepared AgNPs and AuNPs nanoparticles respectively obtained at different concentration of fruit extract. The visual inspection of prepared AgNPs and AuNPs nanoparticles has been shown in Fig. 1(i) and (ii) (inset). These characteristic color variations are due to the excitation of the surface plasmon resonance (SPR) in the metal nanoparticles. Surface plasmon absorption is a small particle effect which is absent in bulk metal. In general, SPR are influenced by size, shape, composition, dispersity, surrounding environment, and surface chemistry. As the effect of increasing fruit extract, the SPR bands of the prepared colloids exhibited blue shift in both AgNPs (451–445 nm) and AuNPs (552–546 nm) with increasing absorbance. The blue shifted and narrow SPR band indicating the formation of small spherical nanoparticles and homogeneous distribution of prepared nanoparticles with the increase of fruit extract was also found in TEM images. The Full Width Half Maximum (FWHM) is reported to be helpful in understanding the particle size and their distribution within the medium. As the concentration of fruit extract increases, an FWHM value decreased in both AgNPs and AuNPs in the reaction medium. In the present case, the particle size of prepared nanoparticles decreases with decreasing FWHM value [22]. The broad SPR bands observed at lower concentration of S. lycopersicums fruit extract are due to large anisotropic particle. At higher concentration of fruit extract the large number of biomolecules present in fruit extract is sufficient to reduce silver and gold ion and forms particle of smaller size [23,24]. As the particles decrease in size, the absorption peak usually shifts toward the blue wavelengths, higher frequency and energies [25]. In the present case, the SPR bands of the prepared colloids exhibited blue shift in the reaction medium with increasing fruit extract concentration. This result represents that the diameter of the prepared nanoparticles
Fig. 2. X-ray diffraction pattern of (a) S. lycopersicums fruit extract, (b) s1, (c) s3, (d) g1 and (e) g3 (⁄ due to S. lycopersicums fruit extract).
Table 1 The particle size, FWHM, lattice constant and cell volume of prepared nanoparticles. S. No.
s1
s3
g1
g3
Particle size ‘D’ (nm) FWHM ‘b’ (2h) Lattice constant ‘a’ (Å) Volume ‘V’ (Å3)
21.68 0.39 4.1014 68.99
13.39 0.72 4.0963 68.73
16.57 0.47 4.068 67.34
14.40 0.58 4.056 66.72
decrease with increasing concentration of the fruit extract. The symmetric nature of the SPR and the absence of peaks in the longer wavelength region indicate the absence of nanoparticle aggregation. This was also confirmed by the TEM results. These nanoparticle solutions were observed to be stable for a time period of one month. In the present case, the prepared nanoparticle introduced negative charge due to the biomolecules and thus repels the particles away from each other, preventing them from aggregation. XRD studies Fig. 2(a) shows the XRD pattern of the dried S. lycopersicums extract. The observed peaks at 28.4° and 40.4° in 2h range indicates the presence of ascorbic acid (JCPDS 22-1560), malic acid (JCPDS 23-1631) and citric acid (JCPDS 22-1568) in the S. lycopersicums extract. Fig. 2(b) and (c) shows the XRD pattern of the prepared AgNPs. The observed diffraction peaks at 37.98°, 44.17°. 64.38° and 77.3° in the 2h range corresponding to (1 1 1), (2 0 0), (2 2 0)
Fig. 3. FTIR spectrum of (a) S. lycopersicums fruit extract, (b) s3 and (c) g3.
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Fig. 4. TEM images at different magnifications of (a–d) of g1.
and (3 1 1) reflection planes were indexed to the face centered cubic (fcc) structure of metallic silver with space group of Fm3m (JCPDS 04-0783). Fig. 2(d) and (e) shows the XRD pattern of the prepared AuNPs. The peaks observed at 38.2°, 44.4°, 64.6° and 77.6° correspond to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) Bragg’s reflections were in good agreement with fcc structure of metallic gold with space group of Fm-3m (JCPDS: 04-0784). In addition to the Bragg peaks representative of fcc silver and gold nanocrystals, two additional peaks were also observed at 28.5° and 40.1°. These peaks represent the presence of carboxylic acid groups present in the S. lycopersicums extract. The average particle size, FWHM value, lattice constant and cell volume of prepared AgNPs and AuNPs were estimated and tabulated in Table 1. Generally, the breadth of a specific phase of material is directly proportional to the mean crystallite size of that material. The broader peaks indicating the crystallite size is small [26]. This indicates that as the concentration of fruit extract increases, the particle size decreased with increasing FWHM value. From these XRD pattern, broadening in peaks occur due to the smaller particle size, which reflect the effects of the experimental conditions on the nucleation and growth of the crystal nuclei [27]. FTIR studies FTIR measurements were carried out to identify the possible biomolecules in S. lycopersicums extract responsible for reducing and stabilizing AgNPs and AuNPs. Fig. 3(a) shows the FTIR spectra of dried S. lycopersicums fruit extract. Fig. 3(b) shows the FTIR
spectrum of s3. Fig. 3(c) shows the FTIR spectrum of g3. The prominent peak observed at 3423 cm 1 in the FTIR spectrum of S. lycopersicums extract was downshifted to 3418 and 3398 cm 1 in the FTIR spectrum of g3 and s3 respectively was due to OH stretching of ascorbic, malic and citric acid [28–30]. A broad peak observed around 1600 cm 1 in the FTIR spectrum of S. lycopersicums extract was appeared as a sharp peak at 1574 and 1614 cm 1 in the spectrum of g3 and s3 were due to C@C ring stretching of ascorbic acid, OCO asymmetric stretching of malic acid and C@O stretching of citric acid. Similarly, the asymmetric feature observed at 1102 cm 1 was appeared as a symmetric nature at 1101 and 1040 cm 1 in the spectrum of g3 and s3, was due to CAOAC stretching of ascorbic acid and CAC stretching of malic and citric acid. It was possible that the carboxylic acid groups present in the extract, adsorbed on the surface of silver nanoparticles, possibly leads to the reduction of Ag+ to Ag0 state. Another interesting peak observed at 1380 cm 1 in the spectrum of extract was observed as a symmetric peak at 1382 and 1397 cm 1 in the spectrum of g3 and s3, was due to OCO symmetric stretching of malic acid, COH deformation of citric acid and CH2 wagging of ascorbic acid. These observed symmetric peaks were also assigned to the ACAO stretching vibration modes of phytochemicals like water soluble components such as phenolic compounds including flavonoids, alkaloids and antioxidant vitamins. It was possible that the (lycopene) carotenoids and polyphenols present in the fruit extract, adsorbed on the surface of nanoparticles. This may thereby supposed to cap the obtained nanoparticles, restrict the agglomeration and enhance the stability.
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Fig. 5. TEM images at different magnifications (a–e) and EDS spectrum (f) of g3.
TEM analysis Fig. 4 shows TEM analysis of g1 showed the formation of irregular contoured nanoparticles with a broad size distribution in the range 27 nm–44 nm with average size of 33 nm. The less concentration of extract concentration at the reaction was responsible for the formation of few larger particles. TEM analysis of g3 showed the formation of smaller nanoparticles of more uniformly sized spherical and little nanotriangles in the range 5 nm–19 nm with an average of 14 nm shown in Fig. 5. Nanotriangles are commonly observed structure in chemically and biologically
synthesized AuNPs [31–36]. The nanotriangles formed as a result of rapid reduction, assembly and room temperature sintering of spherical nanoparticles, rearrangement and aggregation of smaller size particles. The formation of the gold nanotriangles act as a nuclei for further growth into anisotropic triangular structures. The twined particles also observed in Fig. 5(e). Fcc structured metallic nanocrystals have a tendency to nucleate and grow into twinned particles with their surfaces bounded by lowest energy facets (1 1 1) [37]. Fig. 5(e) shows the distance of 0.231 nm and 0.14 nm between lattice planes is in agreement with the (1 1 1) and (2 2 0) lattice spacing of face centered cubic (fcc) gold.
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Fig. 6. TEM images at different magnifications (a–c) and EDS spectrum (d) of s3.
The TEM image of s3 showed the formation of monodispersed and spherical nanoparticles in the range 14 –33 nm with average size of 12 nm observed at Fig. 6(a–c). The distance of 0.238 nm between lattice planes is in agreement with the (1 1 1) lattice spacing of face centered cubic (fcc) silver shown in Fig. 6(c). Although continuous strong interaction between protective biomolecules and surface of nanoparticles fulfilled at s3 and they protect the spherical nanoparticles from sintering and provide size reduction of spherical nanoparticles also. The formation of silver and gold atoms in the prepared nanoparticles was further confirmed by the analysis of the EDX of g3 and s3 shown in Fig. 5(f) and Fig. 6(d). Antibacterial activity Upon studying the antibacterial activity of nanoparticles, the prepared gold (g3) and silver (s3) nanoparticles were tested against Gram positive pathogen Staphylococcus aureus and Gram negative pathogen Pseudomonas aeruginosa. Zone of inhibition around AgNPs (s3) for individual bacterial culture was shown in Fig. 7(a). Zone of inhibition around g3 for individual bacterial culture was shown in Fig. 7(b). Fig. 7(c) also shows that the prepared AgNPs and AuNPs are effective in inhibiting the growth of both gram positive and gram negative bacteria, since significant antibacterial activity is found in all the bacteria. Its biocidal activity is found to be highest for S. aureus. The differences observed in the diameter of zone of inhibition may be due to the difference in the susceptibility of different bacteria to the prepared nanoparticles.
The effective interaction against gram-positive S. aureus was also due to absence of outer membrane in the cell wall. While comparing the antibacterial activity of prepared AgNPs and AuNPs, AgNPs showing more antibacterial activity than AuNPs, because of its larger specific surface area, smaller size and spherical shape. The antibacterial activities of colloidal nanoparticles are influenced by the dimensions of the particles. AgNPs and AuNPs may attach to the surface of the cell membrane and disturb its power function such as permeability and respiration. It is responsible to state that the binding of the particles to the bacteria depends on the surface area available for interaction. Smaller particles having the larger surface area available for interaction will give more bactericidal effect than the larger particles [38]. Effective antibacterial agents should be toxic to different pathogenic bacteria with the ability to be coated as antimicrobial coating on variety of surfaces like wound dressings, medical appliances, biomaterials, purifying and purity testing devices, textiles, biomedical and food packaging, consumer products and so on. The biological and physiochemical properties of this green synthesized AgNPs and AuNPs shows potential as antimicrobial agent which may used in water purification as well as in other biomedical applications. Sensing activity To investigate the interaction of prepared AgNPs (s3) with various alkali metal (Li+, K+, Fe3+) and transition metal ions (Ni2+, Mn2+, Cu4+, Zn2+, Hg2+, Cd2+), 0.2 ml of (3 mM) salts of these
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Fig. 7. Zone of inhibition of (a) s3 and (b) g3 for (i) S. aureus and (ii) P. aeruginosa respectively and (c) antimicrobial activity of s3 and g3 against pathogens.
metals were added into 3 ml of AgNPs by drop by drop and stirred for 2 min. The photographs (Fig. 8(a)) and UV–vis spectra (Fig. 8(a) (inset)) of AgNPs were taken immediately after addition of metal ions, after 2 min of interaction. It was observed that except Fe3+ no other metal ions exhibited a colour change. UV–vis spectra of these heavy metals interacted with AgNPs were shown in
Fig. 8(a). It was observed that for Fe3+ gave 1pale yellow colour and there was no prominent SPR peak, indicating the prepared AgNPs were sensitive and selective towards Fe3+. 1 For interpretation of color in Fig. 8, the reader is referred to the web version of this article.
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To detect the sensitivity of this method in water using optical characteristics of AgNPs, various concentrations of aqueous solution of Fe3+ ions was added into the aqueous AgNPs (3 ml) at room temperature and their UV–vis absorption spectra were recorded at room temperature and shown in Fig. 8(b). Changes in the absorbance strength at peak position were monitored. Upon addition of 124.62 lM Fe3+ solution to AgNPs solution, the reddish brown color disappeared and the solution became colorless and there was no surface plasmon resonance peak for this higher concentration, shown in Fig. 8(b). So the concentration of Fe3+ was limited to 115.36 lM with notable surface plasmon resonance band. After addition of Fe3+ to the green synthesized Ag NPs, Fe3+ ions bounds to the Ag NPs surface to move biological compounds from S. lycopersicums fruit extract away from the silver surface; as a result, a redox reaction between silver and iron ions would be occurred. It was also observed that the optical absorbance intensity reduces and band broadens gradually with the increase of concentration of Fe3+ ions. The main reason for SPR broadening is electron surface scattering which may be enhanced for very small clusters [39]. The observed band broadening of the surface plasmon band reveals that the iron atoms may bind on the silver surface. The linear variation of absorbance (A445 nm) changes and the concentration of Fe3+ over the range from 9.96 lM to 124.62 lM shown in Fig. 8(c). This plot can be fit by a linear equation y = 0.0015x + 0.966, R2 = 0.962. A
good linear correlation, absorbance versus concentration of Fe3+, and the sensitivity of the system towards analyte concentration was found to be 0.0015/lM as measured from the plot of absorbance versus concentration of Fe3+. To investigate the response of green synthesized AuNPs (g3) to various metal ions (Li+, K+, Fe3+, Ni2+, Mn2+, Cu4+, Zn2+, Hg2+, Cd2+), 0.5 ml (5 mM) of stock solutions of the salts of these metals were added into the 4 ml of AuNPs solution. Upon interaction of AuNPs with various metal ions, the colour of Cu4+ solution changed from purple to blue, while the colour changes of other metal ions are negligible. UV–vis spectra of interacted various heavy metals with AuNPs was shown in Fig. 9(a). It was observed that the SPR bands shifted to the red end and absorbance of the SPR bands reduced for all metal ions as compared to that of the AuNPs. Fig. 9(a) clearly indicates that Cu4+ only reveal the presence of a peak at around 800 nm. In addition, Ni2+ also gave secondary peak but unlike
Fig. 8. (a) UV–vis absorption spectrum and photographs (inset) of AgNPs with different heavy metal ions, (b) UV–vis absorption spectrum of AgNPs solution upon addition of Fe3+ ions (9.96–124.62 lM) and (c) plot of absorbance intensity at 445 nm versus Fe3+ concentration.
Fig. 9. (a) UV–vis absorption spectrum and photographs (inset) of AuNPs with different heavy metal ions, (b) UV–vis absorption spectrum of AgNPs solution upon addition of Cu4+ ions (2.2–7.4 mM) and (c) plot of absorbance intensity at A803 nm/ A546 nm versus Cu4+ concentration.
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Cu4+ there was no colour change. This approach has high selective and sensitive to Cu4+. To evaluate the sensitivity of this method various concentration of aqueous solution of Cu4+ ions was added into the aqueous AuNPs (5 ml) at room temperature. Fig. 9(b) shows the UV–vis absorption spectrum for activity of AuNPs as copper sensor. When 1 mM copper sulphate added to AgNPs, the colour of the nanoparticle solution did not change, only the addition of 2.2 mM copper sulphate causes colour changes. This indicates that the analyte directed aggregation of nanoparticles taken place only after the addition of 2.2 mM Cu4+. With increasing concentration from 2.2 mM to 7.07 mM, the intensity of the SPR peak centered at 546 nm degreases gradually as a new SPR band appears at 803 nm with increasing intensity. The presence of new SPR peak was due to the adsorption of molecules causes changes in dielectric environment around a nanoparticle or agglomerated particles. When copper sulphate added to the prepared nanoparticles, Cu4+ ions interact with the biomolecules in the extract on the surface of the nanoparticles form bonds among nanoparticles with Cu4+ ions performing as link for binding sites of biomolecules and eliminating it away from the surface of the nanoparticle surface, in that way aggregation of nanoparticles had taken place. The addition of 2.2 –7.4 mM Cu4+ to the AuNPs solution causes color changes from reddish brown to light blue were observed shown in Fig. 9(b) (inset). The observed color changes and absorbance changes after the addition of 7.07 mM Cu4+ were negligible, suggesting the formation of stable aggregates. So the concentration of copper was limited to 7.07 mM with notable surface plasmon resonance band. The variations of absorbance at 546 nm and 803 nm shown in Fig. 9(c). The linear variation of absorbance (A803nm/A546nm) changes and the concentration of Cu4+ over the range from 2.2 mM to 7.07 mM shown in Fig. 9(c). This plot can be fit by a linear equation y = 0.867x 2.239; R2 = 0.980. A good linear correlation, absorbance versus concentration of Cu4+, and the sensitivity of the system towards analyte concentration was found to be 0.867 mM as measured from the plot of absorbance versus concentration of Cu4+. Hence the AgNPs and AuNPs prepared by green synthesis method can play a role as a surface plasmon resonance based sensor to detect the concentration of iron and copper, which is otherwise a spectroscopically silent heavy metal in water. Conclusion The present study was designed to synthesis gold and AgNPs using fruit extract of S. lycopersicums as reducing agent. Using green synthesis method monodispersed spherical AgNPs with average size of 12 nm and AuNPs with average size of 14 nm have prepared. The prepared nanoparticles were characterized by UV–visible, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) technique to identify the size, shape of nanoparticles and biomolecules act as reducing agents. FTIR measurements show that carboxylic acid groups present in S. lycopersicums extract was used as reducing agent. The prepared AgNPs and AuNPs were stable for one month without aggregation. The surface plasmon resonance of prepared AgNPs and AuNPs was confirmed by UV–visible spectral analysis. The prepared nanoparticles reveal good antimicrobial activity against S. aureus and P. aeruginosa, which are found in water. The prepared nanoparticles were used for sensing ions of heavy metals like Fe3+ and Cu4+ in water using
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a SPR optical sensor. The prepared nanoparticles have found applications water purification processes for inhibiting the growth of bacteria and detecting the concentration of contaminant iron and copper. Acknowledgement The authors are thankful to DST-CURIE New Delhi, UGC-DAE CSR Indore for financial assistance. References [1] Y.W. Lin, C.C. Huangb, H.T. Chang, Analyst 136 (2011) 863–871. [2] M.R. Ganjali, A. Ghafarloo, F. Faridbod, P. Norouzi, Int. J. Electrochem. Sci. 7 (2012) 3706–3716. [3] A. Sugunan, C. Thanachayanont, J. Dutta, J.G. Hilborn, Sci. Technol. Adv. Mater. 6 (2005) 335–340. [4] A. Ravindran, V. Mani, N. Chandrasekaran, A. Mukherjee, Talanta 85 (2011) 533–540. [5] S. Pandey, G.K. Goswami, K.K. Nanda, Int. J. Biol. Macromol. 51 (2012) 583–589. [6] K. Farhadi, M. Forough, R. Molaei, S. Hajizadeh, A. Rafipour, Sens. Actuators, B 161 (2012) 880–885. [7] M.R. Bindhu, M. Umadevi, Spectrochim. Acta A 121 (2014) 596–604. [8] Z. Qi, H. Zhou, N. Matsuda, I. Honma, K. Shimada, A. Takatsu, K. Kato, J. Phys. Chem. B 108 (2004) 7006–7011. [9] M. Umadevi, S. Shalini, M.R. Bindhu, Adv. Nat. Sci. Nanosci. Nanotechnol. 3 (025008) (2012) 1–6. [10] M. Umadevi, M.R. Bindhu, V. Sathe, J. Mater. Sci. Technol. 29 (2013) 317–322. [11] M.R. Bindhu, M. Umadevi, Spectrochim. Acta A 101 (2013) 184–190. [12] M.R. Bindhu, V.G. Sathe, M. Umadevi, Spectrochim. Acta A 115 (2013) 409– 415. [13] Daizy Philip, C. Unni, S. Aswathy Aromal, V.K. Vidhua, Spectrochim. Acta A 78 (2011) 899–904. [14] D. Philip, Spectrochim. Acta A 73 (2009) 374–381. [15] M. Meena Kumari, D. Philip, Spectrochim. Acta A 111 (2013) 154–160. [16] S. Aswathy Aromal, V.K. Vidhu, D. Philip, Spectrochim. Acta A 85 (2012) 99– 104. [17] R. Ulrich, The Biochemistry of Fruits and Their Products, Organic Acids, Academic Press, London, New York, 1970. pp. 89–118. [18] Z. Polívková, P. Šmerák, H. Demová, M. Houška, J. Med. Food 13 (2010) 1443– 1450. [19] F. Shidfar, N. Froghifar, M. Vafa, A. Rajab, S. Hosseini, S. Shidfar, M. Gohari, Int. J. Food Sci. Nutr. 62 (2011) 289–294. [20] A.V. Rao, B. Balachandran, Nutr. Neurosci. 5 (2002) 291–309. [21] M. Ramakrishna, V. Suresh, RSC Adv. 2 (2012) 9914–9920. [22] K.R. Brown, D.G. Walter, M. Natan, Chem. Mater. 12 (2000) 306–313. [23] V. Parashar, R. Parashar, B. Sharma, A.C. Pandey, Dig. J. Nanomater. Biostruct. 4 (2009) 45. [24] M.M. Ganesh Babu, P. Gunasekaran, Colloids Surf. B 74 (2009) 191–195. [25] S.L. Smitha, K.M. Nissamudeen, D. Philip, K.G. Gopchandran, Spectrochim. Acta A 71 (2008) 186–190. [26] A. Irshad, Wani, AparnaGanguly, Jahangeer Ahmed, Tokeer Ahmad, Mater. Lett. 65 (2011) 520–522. [27] A. Becheri, M. Durr, P.L. Nostro, P. Baglioni, J. Nanopart. Res. 10 (2008) 679– 689. [28] C.Y. Panicker, H.T. Varghese, D. Philip, Spectrochim. Acta A 65 (2006) 802–804. [29] S.L. Smitha, K.M. Nissamudeen, D. Philip, K.G. Gopchandran, Spectrochim. Acta Part A 71 (2008) 186–190. [30] J.L. Castro, M.R. Lo´pez-Ramı´rez, J.F. Arenas, J.C. Otero, Vib. Spectrosc. 39 (2005) 240–243. [31] Y. Shao, Y. Jin, S. Dong, Chem. Commun. 9 (2004) 1104–1105. [32] W.T.S. Huck, N. Bowden, P. Onck, T. Pardoen, J.W. Hutchinson, G.M. Whitesides, Langmuir 16 (2000) 3497–3501. [33] R. Geethalakshmi, D.V.L. Sarada, Int. J. Nanomedicine 7 (2012) 5375–5384. [34] J. Huang, Q. Li, D. Sun, Y. Lu, Y. Su, X. Yang, H. Wang, Y. Wang, W. Shao, N. He, J. Hong, C. Chen, Nanotechnology 18 (105104) (2007) 1–11. [35] A. Rai, A. Singh, A. Ahmad, M. Sastry, Langmuir 22 (2006) 736–741. [36] S.S. Shankar, A. Rai, B. Ankamwar, A. Singh, A. Ahmad, M. Sastry, Nat. Mater. 3 (2004) 482–488. [37] J.G. Allpress, J.V. Sanders, Surf. Sci. 7 (1967) 1–25. [38] C. Baker, A. Pradhan, L. Pakstis, J. Pochan Darrin, S.S. Ismat, J. Nanosci. Nanotechnol. 5 (2005) 244–249. [39] S. Link, M.A. El-Sayed, Inter. Rev. Phys. Chem. 19 (2000) 409–453.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Silver and gold nanoparticles for sensor and antibacterial applications M.R. Bindhu, M. Umadevi ⇑ Department of Physics, Mother Teresa Women’s University, Kodaikanal 624101, Tamil Nadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Ag and Au nanoparticles synthesized
using Solanum lycopersicums extract as reducing agent. Shows good antimicrobial activity. Act as very good copper and iron sensors. Can be used in water purification processes.
a r t i c l e
i n f o
Article history: Received 28 November 2013 Received in revised form 4 February 2014 Accepted 19 February 2014 Available online 12 March 2014 Keywords: Silver nanoparticles Gold nanoparticles Solanum lycopersicums Sensing activity Antimicrobial activity
a b s t r a c t Green biogenic method for the synthesis of gold and silver nanoparticles using Solanum lycopersicums extract as reducing agent was studied. The biomolecules present in the extract was responsible for reduction of Au3+ and Ag+ ions from HAuCl4 and AgNO3 respectively. The prepared nanoparticles were characterized by UV–visible spectroscopy (UV–vis), Fourier transform infrared spectroscopy (FTIR), Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) technique to identify the size, shape of nanoparticles and biomolecules act as reducing agents. UV–visible spectra show the surface plasmon resonance peak at 546 nm and 445 nm corresponding to gold and silver nanoparticles respectively. Crystalline nature of the nanoparticles was evident from TEM images and XRD analysis. TEM images showed average size of 14 nm and 12 nm for prepared gold and silver nanoparticles respectively. FTIR analysis provides the presence of biomolecules responsible for the reduction and stability of the prepared silver and gold nanoparticles. XRD analysis of the silver and gold nanoparticles confirmed the formation of metallic silver and gold. The prepared gold and silver nanoparticles show good sensing and antimicrobial activity. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Optical sensor based on surface plasmon resonance for detection of heavy metals in water is one of the most sensitive methods which will be advantageous over other techniques as it will be a simple, inexpensive and fast method. In the past years a number of nanoparticles based sensor have been reported [1–4]. Recently ⇑ Corresponding author. Tel.: +91 04542241685; fax: +91 4542 241122. E-mail address: [email protected] (M. Umadevi). http://dx.doi.org/10.1016/j.saa.2014.02.119 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
green synthesized silver nanoparticles used in an optical sensor based on localized SPR for ammonia, mercury, copper and zinc detection was studied [5–7]. Water being the prime vital survival need for living beings in this world. Pure water is absolutely essential for a safe, healthy and long life. Purity of water is polluted by the existence of pathogenic bacteria and carcinogenic chemicals present in it. Presence of heavy metals such as cadmium (Cd), Zinc (Zn), mercury (Hg), arsenic (As), silver (Ag), chromium (Cr), copper (Cu), iron (Fe), and the platinum group elements in water causes health hazards. In particular the detection of Cu4+ and Fe3+ in water
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has been of great research interest due to the important role of Fe and Cu plays in the biological processes within the human body. Drinking water can be a source of Cu to humans as a result of water treatment, usage of galvanized pipes and tanks in distribution systems. So, detection and control of Cu ions in water samples is very important. Iron is usually found in its oxidized form, which is insoluble and nondegradable. As a result, low solubility of iron does not give the necessary sensitivity for detection of this metal ion in water. So, detection and control of Cu and Fe ions in water samples is very important. Microorganisms causing diseases that characteristically are waterborne, significantly contain protozoa and bacteria. The removal or inactivation of pathogenic microorganisms is the important step in the treatment of wastewater. In the present study, we have designed simple device to detect the contaminant concentration in water as well as to inhibit the growth of different pathogenic bacteria. Metal nanoparticles have been studied extensively because of their applications in areas such as optics, optoelectronics, catalysis, photography, nanostructure fabrication, chemical/biochemical sensing, photonics and surface enhanced Raman scattering [8]. Among the known nanoparticles, silver and gold has been widely studied because of their unique optical, electrical, and photothermal properties. Silver (AgNPs) and gold (AuNPs) nanoparticles can be synthesized using chemical and physical methods, which are expensive and often involve the use of toxic, hazardous chemicals which may pose environmental risks. A great deal of attempt has been put into the green synthesis of metal nanoparticles, using plants are a simple and viable alternative to chemical procedures and physical methods. Biosynthesis of nanoparticles using Daucus carota, Solanum lycopersicums, Hibiscus cannabinus leaf, Moringa oliefera flower, Murraya Koenigii leaf, mushroom, coconut oil, Macrotyloma uniflorum and Ananas comosus has been reported [9–16,7]. In this work, AgNPs and AuNPs were synthesized using S. lycopersicums fruit extract as reducing agent. Since S. lycopersicums is a commonly available fruit and good source of citric acid, malic acid and ascorbic acid [17]. S. lycopersicums also contains a variety of phytochemicals, including carotenoids and polyphenols. Lycopene, phytoene, phytofluene, flavonoids, quercetin, kaempferol and the provitamin A carotenoids beta-carotene are the important carotenoids in S. lycopersicums. Many of these nutrients and phytochemicals have antioxidant properties and in combination with lycopene may contribute to the numerous health benefits of S. lycopersicums. S. lycopersicums has anticancer properties [18]. S. lycopersicums consumption might be beneficial for reducing cardiovascular risk associated with type 2 diabetes [19] and protective against neurodegenerative diseases [20]. The disposal of the S. lycopersicums skin and its other fibrous materials is an economic waste for many food processing industries. Ramakrishna et al. reported that S. lycopersicums peels can effectively remove different contaminants in water, including dissolved organic and inorganic chemicals, dyes and pesticides, and they can also be used in large scale applications [21]. The synthesis of well dispersed silver nanoparticles using S. lycopersicums fruit extract as reducing agent has been reported [10]. In the present study, the synthesis and characterization of monodispersed AgNPs and AuNPs using fruit extract of S. lycopersicums has been described. The sensing and antibacterial activity of these prepared AgNPs and AuNPs has been also described.
collected from local supermarket in Kodaikanal, Tamilnadu, India. 100 g of washed S. lycopersicums were crushed in a mixer grinder for extraction. The extract was separated by centrifugation at 1000 rpm for 10 min to remove insoluble fractions and macromolecules. Then the light yellow extract was collected for further experiments. An aqueous solution of HAuCl4 (3 mM) was added to different concentration of fruit extract (5, 10 and 15 ml) and stirred for 5 min at room temperature (g1, g2 and g3). During the synthesis, initially becomes colorless and turned into brownish purple indicating the formation of gold colloid. Similarly, an aqueous solution of AgNO3 (3 mM) was added to different concentration of fruit extract (5, 10 and 15 ml) and stirred for 5 min at room temperature (s1, s2 and s3). Upon addition of the extract, the colorless solution changes colour from light yellow to reddish orange. The absorption spectra of the prepared nanoparticles were measured using a Shimadzu spectrophotometer (UV 1700) in 300– 800 nm range. XRD analysis of the prepared nanoparticles was done using PANalytical X’pert – PRO diffractometer with Cu Ka radiation operated at 40 kV/30 mA. FTIR measurements were obtained on a Nexus 670 FTIR instrument with the sample as KBr pellets. Transmission Electron Microscopic (TEM) analysis was done using a JEOL JEM 2100 High Resolution Transmission Electron Microscope operating at 200 kV.
Experimental details Chloroauric acid, silver nitrate, copper sulphate, ferric chloride, nickel nitrate, potassium chloride, cadmium acetate, manganous acetate, mercuric iodide, lithium hydroxide and zinc acetate were obtained from Sigma Aldrich Chemicals. S. lycopersicums fruit was
Fig. 1. Optical absorption spectra for S. lycopersicums fruit extract concentration variation study of (i) prepared gold nanoparticle (inset: colour changes of the gold colloid) and (ii) prepared AgNPs (inset: colour changes of the silver colloid) (a, b and c versus 5, 10 and 15 ml respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Results and discussion Optical studies Noble metal nanoparticles exhibit a strong absorption band in the visible region, which originates from coherent excitation of the conduction band electrons induced by the interacting electromagnetic field. UV–visible absorption spectra have been proved to be quite sensitive to the formation of silver and gold colloids because silver and gold nanoparticles exhibit an intense absorption peak due to the surface plasmon excitation. Fig. 1(i) and (ii) shows the optical absorption spectra of prepared AgNPs and AuNPs nanoparticles respectively obtained at different concentration of fruit extract. The visual inspection of prepared AgNPs and AuNPs nanoparticles has been shown in Fig. 1(i) and (ii) (inset). These characteristic color variations are due to the excitation of the surface plasmon resonance (SPR) in the metal nanoparticles. Surface plasmon absorption is a small particle effect which is absent in bulk metal. In general, SPR are influenced by size, shape, composition, dispersity, surrounding environment, and surface chemistry. As the effect of increasing fruit extract, the SPR bands of the prepared colloids exhibited blue shift in both AgNPs (451–445 nm) and AuNPs (552–546 nm) with increasing absorbance. The blue shifted and narrow SPR band indicating the formation of small spherical nanoparticles and homogeneous distribution of prepared nanoparticles with the increase of fruit extract was also found in TEM images. The Full Width Half Maximum (FWHM) is reported to be helpful in understanding the particle size and their distribution within the medium. As the concentration of fruit extract increases, an FWHM value decreased in both AgNPs and AuNPs in the reaction medium. In the present case, the particle size of prepared nanoparticles decreases with decreasing FWHM value [22]. The broad SPR bands observed at lower concentration of S. lycopersicums fruit extract are due to large anisotropic particle. At higher concentration of fruit extract the large number of biomolecules present in fruit extract is sufficient to reduce silver and gold ion and forms particle of smaller size [23,24]. As the particles decrease in size, the absorption peak usually shifts toward the blue wavelengths, higher frequency and energies [25]. In the present case, the SPR bands of the prepared colloids exhibited blue shift in the reaction medium with increasing fruit extract concentration. This result represents that the diameter of the prepared nanoparticles
Fig. 2. X-ray diffraction pattern of (a) S. lycopersicums fruit extract, (b) s1, (c) s3, (d) g1 and (e) g3 (⁄ due to S. lycopersicums fruit extract).
Table 1 The particle size, FWHM, lattice constant and cell volume of prepared nanoparticles. S. No.
s1
s3
g1
g3
Particle size ‘D’ (nm) FWHM ‘b’ (2h) Lattice constant ‘a’ (Å) Volume ‘V’ (Å3)
21.68 0.39 4.1014 68.99
13.39 0.72 4.0963 68.73
16.57 0.47 4.068 67.34
14.40 0.58 4.056 66.72
decrease with increasing concentration of the fruit extract. The symmetric nature of the SPR and the absence of peaks in the longer wavelength region indicate the absence of nanoparticle aggregation. This was also confirmed by the TEM results. These nanoparticle solutions were observed to be stable for a time period of one month. In the present case, the prepared nanoparticle introduced negative charge due to the biomolecules and thus repels the particles away from each other, preventing them from aggregation. XRD studies Fig. 2(a) shows the XRD pattern of the dried S. lycopersicums extract. The observed peaks at 28.4° and 40.4° in 2h range indicates the presence of ascorbic acid (JCPDS 22-1560), malic acid (JCPDS 23-1631) and citric acid (JCPDS 22-1568) in the S. lycopersicums extract. Fig. 2(b) and (c) shows the XRD pattern of the prepared AgNPs. The observed diffraction peaks at 37.98°, 44.17°. 64.38° and 77.3° in the 2h range corresponding to (1 1 1), (2 0 0), (2 2 0)
Fig. 3. FTIR spectrum of (a) S. lycopersicums fruit extract, (b) s3 and (c) g3.
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Fig. 4. TEM images at different magnifications of (a–d) of g1.
and (3 1 1) reflection planes were indexed to the face centered cubic (fcc) structure of metallic silver with space group of Fm3m (JCPDS 04-0783). Fig. 2(d) and (e) shows the XRD pattern of the prepared AuNPs. The peaks observed at 38.2°, 44.4°, 64.6° and 77.6° correspond to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) Bragg’s reflections were in good agreement with fcc structure of metallic gold with space group of Fm-3m (JCPDS: 04-0784). In addition to the Bragg peaks representative of fcc silver and gold nanocrystals, two additional peaks were also observed at 28.5° and 40.1°. These peaks represent the presence of carboxylic acid groups present in the S. lycopersicums extract. The average particle size, FWHM value, lattice constant and cell volume of prepared AgNPs and AuNPs were estimated and tabulated in Table 1. Generally, the breadth of a specific phase of material is directly proportional to the mean crystallite size of that material. The broader peaks indicating the crystallite size is small [26]. This indicates that as the concentration of fruit extract increases, the particle size decreased with increasing FWHM value. From these XRD pattern, broadening in peaks occur due to the smaller particle size, which reflect the effects of the experimental conditions on the nucleation and growth of the crystal nuclei [27]. FTIR studies FTIR measurements were carried out to identify the possible biomolecules in S. lycopersicums extract responsible for reducing and stabilizing AgNPs and AuNPs. Fig. 3(a) shows the FTIR spectra of dried S. lycopersicums fruit extract. Fig. 3(b) shows the FTIR
spectrum of s3. Fig. 3(c) shows the FTIR spectrum of g3. The prominent peak observed at 3423 cm 1 in the FTIR spectrum of S. lycopersicums extract was downshifted to 3418 and 3398 cm 1 in the FTIR spectrum of g3 and s3 respectively was due to OH stretching of ascorbic, malic and citric acid [28–30]. A broad peak observed around 1600 cm 1 in the FTIR spectrum of S. lycopersicums extract was appeared as a sharp peak at 1574 and 1614 cm 1 in the spectrum of g3 and s3 were due to C@C ring stretching of ascorbic acid, OCO asymmetric stretching of malic acid and C@O stretching of citric acid. Similarly, the asymmetric feature observed at 1102 cm 1 was appeared as a symmetric nature at 1101 and 1040 cm 1 in the spectrum of g3 and s3, was due to CAOAC stretching of ascorbic acid and CAC stretching of malic and citric acid. It was possible that the carboxylic acid groups present in the extract, adsorbed on the surface of silver nanoparticles, possibly leads to the reduction of Ag+ to Ag0 state. Another interesting peak observed at 1380 cm 1 in the spectrum of extract was observed as a symmetric peak at 1382 and 1397 cm 1 in the spectrum of g3 and s3, was due to OCO symmetric stretching of malic acid, COH deformation of citric acid and CH2 wagging of ascorbic acid. These observed symmetric peaks were also assigned to the ACAO stretching vibration modes of phytochemicals like water soluble components such as phenolic compounds including flavonoids, alkaloids and antioxidant vitamins. It was possible that the (lycopene) carotenoids and polyphenols present in the fruit extract, adsorbed on the surface of nanoparticles. This may thereby supposed to cap the obtained nanoparticles, restrict the agglomeration and enhance the stability.
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Fig. 5. TEM images at different magnifications (a–e) and EDS spectrum (f) of g3.
TEM analysis Fig. 4 shows TEM analysis of g1 showed the formation of irregular contoured nanoparticles with a broad size distribution in the range 27 nm–44 nm with average size of 33 nm. The less concentration of extract concentration at the reaction was responsible for the formation of few larger particles. TEM analysis of g3 showed the formation of smaller nanoparticles of more uniformly sized spherical and little nanotriangles in the range 5 nm–19 nm with an average of 14 nm shown in Fig. 5. Nanotriangles are commonly observed structure in chemically and biologically
synthesized AuNPs [31–36]. The nanotriangles formed as a result of rapid reduction, assembly and room temperature sintering of spherical nanoparticles, rearrangement and aggregation of smaller size particles. The formation of the gold nanotriangles act as a nuclei for further growth into anisotropic triangular structures. The twined particles also observed in Fig. 5(e). Fcc structured metallic nanocrystals have a tendency to nucleate and grow into twinned particles with their surfaces bounded by lowest energy facets (1 1 1) [37]. Fig. 5(e) shows the distance of 0.231 nm and 0.14 nm between lattice planes is in agreement with the (1 1 1) and (2 2 0) lattice spacing of face centered cubic (fcc) gold.
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Fig. 6. TEM images at different magnifications (a–c) and EDS spectrum (d) of s3.
The TEM image of s3 showed the formation of monodispersed and spherical nanoparticles in the range 14 –33 nm with average size of 12 nm observed at Fig. 6(a–c). The distance of 0.238 nm between lattice planes is in agreement with the (1 1 1) lattice spacing of face centered cubic (fcc) silver shown in Fig. 6(c). Although continuous strong interaction between protective biomolecules and surface of nanoparticles fulfilled at s3 and they protect the spherical nanoparticles from sintering and provide size reduction of spherical nanoparticles also. The formation of silver and gold atoms in the prepared nanoparticles was further confirmed by the analysis of the EDX of g3 and s3 shown in Fig. 5(f) and Fig. 6(d). Antibacterial activity Upon studying the antibacterial activity of nanoparticles, the prepared gold (g3) and silver (s3) nanoparticles were tested against Gram positive pathogen Staphylococcus aureus and Gram negative pathogen Pseudomonas aeruginosa. Zone of inhibition around AgNPs (s3) for individual bacterial culture was shown in Fig. 7(a). Zone of inhibition around g3 for individual bacterial culture was shown in Fig. 7(b). Fig. 7(c) also shows that the prepared AgNPs and AuNPs are effective in inhibiting the growth of both gram positive and gram negative bacteria, since significant antibacterial activity is found in all the bacteria. Its biocidal activity is found to be highest for S. aureus. The differences observed in the diameter of zone of inhibition may be due to the difference in the susceptibility of different bacteria to the prepared nanoparticles.
The effective interaction against gram-positive S. aureus was also due to absence of outer membrane in the cell wall. While comparing the antibacterial activity of prepared AgNPs and AuNPs, AgNPs showing more antibacterial activity than AuNPs, because of its larger specific surface area, smaller size and spherical shape. The antibacterial activities of colloidal nanoparticles are influenced by the dimensions of the particles. AgNPs and AuNPs may attach to the surface of the cell membrane and disturb its power function such as permeability and respiration. It is responsible to state that the binding of the particles to the bacteria depends on the surface area available for interaction. Smaller particles having the larger surface area available for interaction will give more bactericidal effect than the larger particles [38]. Effective antibacterial agents should be toxic to different pathogenic bacteria with the ability to be coated as antimicrobial coating on variety of surfaces like wound dressings, medical appliances, biomaterials, purifying and purity testing devices, textiles, biomedical and food packaging, consumer products and so on. The biological and physiochemical properties of this green synthesized AgNPs and AuNPs shows potential as antimicrobial agent which may used in water purification as well as in other biomedical applications. Sensing activity To investigate the interaction of prepared AgNPs (s3) with various alkali metal (Li+, K+, Fe3+) and transition metal ions (Ni2+, Mn2+, Cu4+, Zn2+, Hg2+, Cd2+), 0.2 ml of (3 mM) salts of these
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Fig. 7. Zone of inhibition of (a) s3 and (b) g3 for (i) S. aureus and (ii) P. aeruginosa respectively and (c) antimicrobial activity of s3 and g3 against pathogens.
metals were added into 3 ml of AgNPs by drop by drop and stirred for 2 min. The photographs (Fig. 8(a)) and UV–vis spectra (Fig. 8(a) (inset)) of AgNPs were taken immediately after addition of metal ions, after 2 min of interaction. It was observed that except Fe3+ no other metal ions exhibited a colour change. UV–vis spectra of these heavy metals interacted with AgNPs were shown in
Fig. 8(a). It was observed that for Fe3+ gave 1pale yellow colour and there was no prominent SPR peak, indicating the prepared AgNPs were sensitive and selective towards Fe3+. 1 For interpretation of color in Fig. 8, the reader is referred to the web version of this article.
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To detect the sensitivity of this method in water using optical characteristics of AgNPs, various concentrations of aqueous solution of Fe3+ ions was added into the aqueous AgNPs (3 ml) at room temperature and their UV–vis absorption spectra were recorded at room temperature and shown in Fig. 8(b). Changes in the absorbance strength at peak position were monitored. Upon addition of 124.62 lM Fe3+ solution to AgNPs solution, the reddish brown color disappeared and the solution became colorless and there was no surface plasmon resonance peak for this higher concentration, shown in Fig. 8(b). So the concentration of Fe3+ was limited to 115.36 lM with notable surface plasmon resonance band. After addition of Fe3+ to the green synthesized Ag NPs, Fe3+ ions bounds to the Ag NPs surface to move biological compounds from S. lycopersicums fruit extract away from the silver surface; as a result, a redox reaction between silver and iron ions would be occurred. It was also observed that the optical absorbance intensity reduces and band broadens gradually with the increase of concentration of Fe3+ ions. The main reason for SPR broadening is electron surface scattering which may be enhanced for very small clusters [39]. The observed band broadening of the surface plasmon band reveals that the iron atoms may bind on the silver surface. The linear variation of absorbance (A445 nm) changes and the concentration of Fe3+ over the range from 9.96 lM to 124.62 lM shown in Fig. 8(c). This plot can be fit by a linear equation y = 0.0015x + 0.966, R2 = 0.962. A
good linear correlation, absorbance versus concentration of Fe3+, and the sensitivity of the system towards analyte concentration was found to be 0.0015/lM as measured from the plot of absorbance versus concentration of Fe3+. To investigate the response of green synthesized AuNPs (g3) to various metal ions (Li+, K+, Fe3+, Ni2+, Mn2+, Cu4+, Zn2+, Hg2+, Cd2+), 0.5 ml (5 mM) of stock solutions of the salts of these metals were added into the 4 ml of AuNPs solution. Upon interaction of AuNPs with various metal ions, the colour of Cu4+ solution changed from purple to blue, while the colour changes of other metal ions are negligible. UV–vis spectra of interacted various heavy metals with AuNPs was shown in Fig. 9(a). It was observed that the SPR bands shifted to the red end and absorbance of the SPR bands reduced for all metal ions as compared to that of the AuNPs. Fig. 9(a) clearly indicates that Cu4+ only reveal the presence of a peak at around 800 nm. In addition, Ni2+ also gave secondary peak but unlike
Fig. 8. (a) UV–vis absorption spectrum and photographs (inset) of AgNPs with different heavy metal ions, (b) UV–vis absorption spectrum of AgNPs solution upon addition of Fe3+ ions (9.96–124.62 lM) and (c) plot of absorbance intensity at 445 nm versus Fe3+ concentration.
Fig. 9. (a) UV–vis absorption spectrum and photographs (inset) of AuNPs with different heavy metal ions, (b) UV–vis absorption spectrum of AgNPs solution upon addition of Cu4+ ions (2.2–7.4 mM) and (c) plot of absorbance intensity at A803 nm/ A546 nm versus Cu4+ concentration.
M.R. Bindhu, M. Umadevi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 37–45
Cu4+ there was no colour change. This approach has high selective and sensitive to Cu4+. To evaluate the sensitivity of this method various concentration of aqueous solution of Cu4+ ions was added into the aqueous AuNPs (5 ml) at room temperature. Fig. 9(b) shows the UV–vis absorption spectrum for activity of AuNPs as copper sensor. When 1 mM copper sulphate added to AgNPs, the colour of the nanoparticle solution did not change, only the addition of 2.2 mM copper sulphate causes colour changes. This indicates that the analyte directed aggregation of nanoparticles taken place only after the addition of 2.2 mM Cu4+. With increasing concentration from 2.2 mM to 7.07 mM, the intensity of the SPR peak centered at 546 nm degreases gradually as a new SPR band appears at 803 nm with increasing intensity. The presence of new SPR peak was due to the adsorption of molecules causes changes in dielectric environment around a nanoparticle or agglomerated particles. When copper sulphate added to the prepared nanoparticles, Cu4+ ions interact with the biomolecules in the extract on the surface of the nanoparticles form bonds among nanoparticles with Cu4+ ions performing as link for binding sites of biomolecules and eliminating it away from the surface of the nanoparticle surface, in that way aggregation of nanoparticles had taken place. The addition of 2.2 –7.4 mM Cu4+ to the AuNPs solution causes color changes from reddish brown to light blue were observed shown in Fig. 9(b) (inset). The observed color changes and absorbance changes after the addition of 7.07 mM Cu4+ were negligible, suggesting the formation of stable aggregates. So the concentration of copper was limited to 7.07 mM with notable surface plasmon resonance band. The variations of absorbance at 546 nm and 803 nm shown in Fig. 9(c). The linear variation of absorbance (A803nm/A546nm) changes and the concentration of Cu4+ over the range from 2.2 mM to 7.07 mM shown in Fig. 9(c). This plot can be fit by a linear equation y = 0.867x 2.239; R2 = 0.980. A good linear correlation, absorbance versus concentration of Cu4+, and the sensitivity of the system towards analyte concentration was found to be 0.867 mM as measured from the plot of absorbance versus concentration of Cu4+. Hence the AgNPs and AuNPs prepared by green synthesis method can play a role as a surface plasmon resonance based sensor to detect the concentration of iron and copper, which is otherwise a spectroscopically silent heavy metal in water. Conclusion The present study was designed to synthesis gold and AgNPs using fruit extract of S. lycopersicums as reducing agent. Using green synthesis method monodispersed spherical AgNPs with average size of 12 nm and AuNPs with average size of 14 nm have prepared. The prepared nanoparticles were characterized by UV–visible, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) technique to identify the size, shape of nanoparticles and biomolecules act as reducing agents. FTIR measurements show that carboxylic acid groups present in S. lycopersicums extract was used as reducing agent. The prepared AgNPs and AuNPs were stable for one month without aggregation. The surface plasmon resonance of prepared AgNPs and AuNPs was confirmed by UV–visible spectral analysis. The prepared nanoparticles reveal good antimicrobial activity against S. aureus and P. aeruginosa, which are found in water. The prepared nanoparticles were used for sensing ions of heavy metals like Fe3+ and Cu4+ in water using
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